Generally, to manufacture a semiconductor integrated device such as a semiconductor integrated circuit, film-formation, oxidative diffusion, etching and the like are repeatedly applied onto a semiconductor wafer to form numeral transistors, capacitors, and resistances, and thereafter these elements are connected with a wiring pattern. Furthermore, since the development of a high-performance integrated circuit and a multi-functional integrated circuit has been demanded, a further reduction of wiring width and higher integration of the elements are demanded. Moreover, a multi-layered structure has come to be employed in which circuits themselves are stacked one upon the other with an insulating layer interposed between them.
Since a sectional area of the wiring and a connecting portion is reduced under these circumstances, there is a tendency of increasing resistance. It follows that, as a wiring material, copper tends to be used in place of aluminium used generally at present. This is because copper is highly resistant to electromigration and its resistivity is relatively low even though form-formation is not so easy as aluminum.
Usually, as a gate electrode used in a transistor element, a doped polysilicon layer is used alone or a double layered structure electrode is used which is formed by stacking a molybdenum silicide layer or a tungsten silicide layer on the doped polysilicon layer. However, to further reduce the resistivity, an attempt has been made to replace upper silicide layer of the double-layered gate electrode with a single metal layer, for example, a tungsten layer.
Incidentally, copper and tungsten themselves are highly active metals, so that they are likely to react with other elements. For example, metal copper has a large diffusion coefficient, so that it diffuses and segregates into Si, SiO2 or the like, causing defects. As a result, not only resistance value increases but also exfoliation occurs.
When a metal tungsten film is used as one of the layers of the gate electrode of the double layered structure, the silicon atoms of the lower doped polysilicon layer and tungsten of the upper metal tungsten layer are mutually diffused and react with each other to produce tungsten silicide having a large resistivity.
To prevent the reaction between the metal copper and the metal tungsten, it is possible to use a barrier metal such as TiN (titanium nitride) as conventionally used. However, the TiN layer is not a preferable barrier metal because affinity, in other words, adhesiveness, with the metal copper film and the metal tungsten film, is not satisfactory.
Recently, the semiconductor integrated circuit has been desired to be formed in more integrated and more multi-layered structure and operated at a higher speed. To satisfy these demands, it is required to reduce a resistivity of a gate electrode, for example, by reducing the thickness of individual layers and to increase an aspect ratio (a high aspect ratio) during etching processing.
However, if the thickness of a tungsten film constituting the gate electrode is reduced, the tungsten film degrades in adhesiveness to an underlying layer, for example, the polysilicon layer, and in heat resistance. It is also possible herein that a conventionally known TiN film is interposed between both layers as a barrier metal. However, in this case, the adhesiveness between the TiN film and the polysilicon film at the interface degrades, causing exfoliation.